1. Field
The present invention relates to a fixed ball type joint for a vehicle, and more particularly, to a fixed ball type joint for a vehicle, which reduces the friction between a cage and an outer race and friction between the cage and an inner race by changing shapes of an inner spherical diameter of the outer race and an outer spherical diameter of the inner race.
2. Description of the Related Art
In general, a joint functions to transmit rotational power (torque) between two rotation shafts which meet each other at an angle. In the case of a propeller shaft having a small power transmission angle, a hook joint, a flexible joint, etc. are used, and in the case of the driving shaft of a front wheel drive vehicle having a large power transmission angle, a constant velocity joint is used.
Since the constant velocity joint can reliably transmit power at a constant velocity even when an angle between a driving shaft and a driven shaft is large, the constant velocity joint is mainly used for the axle shaft of an independent suspension type front wheel drive vehicle. When viewed from a shaft, a tripod type constant velocity joint is provided to one end of the shaft which faces an engine (i.e., the inboard-side end), and a ball type joint is provided to the other end of the shaft which faces a tire (i.e., the outboard-side end).
FIG. 1 is a cross-sectional view illustrating conventional constant velocity joints, and FIG. 2 is a schematic view illustrating an external appearance of the conventional constant velocity joints shown in FIG. 1.
As shown in FIGS. 1 and 2, the conventional constant velocity joints comprise a tripod type constant velocity joint which is provided to an engine side with respect to a shaft 1 (the inboard-side end) and a fixed ball type joint provided to a wheel side with respect to the shaft 1 (the outboard-side end).
The tripod type joint installed on the engine side with respect to the shaft 1 (the inboard-side end) comprises a housing 2 which transmits rotational power of the engine (not shown) and is defined with track grooves on the inner surface thereof, the shaft 1 which receives the rotational power from the housing 2 and rotates, a spider 3 which is disposed in the housing 2, is coupled to one end of the shaft 1 to connect the housing 2 and the shaft 1 with each other and is formed with three trunnions to be respectively inserted into the track grooves of the housing 2, needle rollers 6 which are arranged on the circumferential outer surface of each trunnion of the spider 3, inner rollers 5 each of which is arranged around the needle rollers 6 for each trunnion of the spider 3, outer rollers 4 each of which is installed on the circumferential outer surface of each inner roller 5 to reduce friction between the housing 2 and the shaft 1, a strike out 7 which is installed on upper ends of the needle rollers 6 and of each inner roller 5, a circlip 8 for preventing the strike out 7 from being dislodged, a boot 10 having one end which is connected to the housing 2 and the other end which is connected to the shaft 1, and clamping bands 11 and 12 which clamp both ends of the boot 10.
In addition, the ball type joint installed on the wheel side with respect to the shaft 1 (the outboard-side end) comprises the shaft 1 which receives the rotational power from the housing 2 and rotates, an inner race 15 which is installed on one end of the shaft 1 to receive the rotational power from the tripod type constant velocity joint and to then rotate, an outer race 13 which is installed around the inner race 15, a plurality of balls 16 for transmitting the rotational power of the inner race 15 to the outer race 13, a cage 14 for supporting the balls 16, a sensor ring 17 which is installed around the outer race 13, a boot 18 having one end which is connected to the shaft 1 and the other end which is connected to the outer race 13, and clamping bands 19 and 20 which clamp both ends of the boot 18.
A damper 21 is installed at the center of the shaft 1 using bands 22 and 23 and has a weight installed in a body.
Hereafter, the operation of the conventional constant velocity joints constructed as mentioned above will be described.
As the rotational power outputted from an engine (not shown) is transmitted to the housing 2 through a transmission (not shown), the housing 2 is rotated. The rotational power of the housing 2 is transmitted to the spider 3 through the outer rollers 4, the inner rollers 5 and the needle rollers 6, and then the shaft 1 to which the spider 3 is coupled is rotated.
In addition, the rotational power of the shaft 1 is transmitted to the outer race 13 through the inner race 15 and the balls 16, and then the wheel (not shown) connected to the outer race 13 is rotated.
In this case, in the tripod type constant velocity joint which is provided to the engine side with respect to the shaft 1 (i.e., the inboard-side end), as the outer rollers 4 slide in the track grooves of the housing 2, the rotation angle of the shaft 1 which is operationally associated with the outer rollers 4 is changed, so that a joint angle is created to follow the displacement of a vehicle. In the ball type constant velocity joint which is provided to the wheel side with respect to the shaft 1 (i.e., the outboard-side end), the rotation angle of the outer race 13 is changed due to the presence of the balls 16, so that a joint angle is created to follow the displacement of the vehicle.
The boot 10 of the tripod type joint and the boot 18 of the ball type joint respectively function to enclose the tripod type joint and the ball type joint, so that the tripod type joint and the ball type joint are prevented from being contaminated by foreign contaminant substances.
In addition, when the torque outputted from the engine and the transmission is transmitted through the shaft 1 to wheels, unbalanced rotation may occur at a certain rotation angle of the shaft 1 rotating at high speed, which may result in undesired vibrations and adversely affect the operation of a drive system. In order to prevent the undesired vibrations due to unbalanced rotation, the damper 21 installed at the center of the shaft 1 may prevent booming noises from occurring to the shaft 1 rotating at high speed due to the detrimental vibration frequency.
FIG. 3 is an exploded view illustrating a conventional ball type constant velocity joint for a vehicle, FIG. 4 is a cross-sectional view illustrating essential parts of the conventional ball type constant velocity joint for a vehicle before a joint angle is created, and FIG. 5 is a cross-sectional view illustrating essential parts of the conventional ball type constant velocity joint after a joint angle is created.
As shown in FIGS. 3 to 5, in the conventional ball type constant velocity joint for a vehicle, the balls 16 are fixed by the cage 14 and the inner race 15. During steering, the balls 16 are moved in a ball track formed on an inner spherical surface of the outer race 13 in a lengthwise direction.
A center O1 of a track radius R11 of the outer race 13 and a center O2 of a track radius R21 of the inner race 15 are symmetrical with each other with regard to a joint rotation center O and are offset by a constant offset amount f in a joint axial direction, that is, in the X-axis direction, to constrain the balls 16.
Here, when the inner race 15 receives a torque, an inner spherical radius R32 of the cage 14 and an outer spherical radius R22 of the inner race 15 come into contact with each other according to dynamic displacement of a steering device and a vehicle, so that a joint angle is created by θ with respect to the joint rotation center O.
In addition, as an outer spherical radius R31 of the cage 14 and an inner spherical radius R12 of the outer race 13 come into contact with each other, a joint angle is created in the cage 14 by an angle θ/2 with respect to the joint rotation center O, so that the balls 16 are constrained by a cage window 143 to be positioned on a Y plane, that is, a plane of the cage 14 located to correspond to a joint angle amount of θ/2.
When the rotational power is transmitted, a contact force Fo is created between a track of the outer race 13 and the balls 16 and a contact force Fi is created between a track of the inner race 15 and the balls 16. A cage axial ball division force Fc is generated by the contact forces Fo and Fi, thereby pushing the balls 16 toward an entrance of the outer race 13.
Accordingly, as frictional resistance is generated due to a component-to-component, a loss in torque transmission efficiency may be caused. The frictional resistance generated on the outer spherical radius R31 of the cage 14 and the inner spherical radius R32 of the cage 14 will now be described in more detail.
FIG. 6 is a perspective view illustrating essential parts of the conventional fixed ball type constant velocity joint for a vehicle to show friction between a cage and an outer race when a joint angle is created.
As shown in FIG. 6, when a joint angle of the inner race 15 is created by θ, contact points of the outer spherical radius R31 of the cage 14 and the inner spherical radius R12 of the outer race 13 are generated at ends of an outer race inner spherical surface length Lo and an outer diameter part of a cage width Lc due to the cage axial ball division force Fc and a tolerance between the two components, that is, between the outer race 13 and the cage 14. Here, frictional forces μF1a and μF1b are generated by contact forces F1a and F1b. The frictional forces μF1a and μF1b are proportional to contact angles θ1a and θ1b. That is to say, if the outer race inner spherical surface length Lo and the cage width Lc are increased, the contact angles θ1a and θ1b are increased and the frictional forces μF1a and μF1b are also increased. Therefore, in order to reduce the frictional forces μF1a and μF1b between the cage 14 and the outer race 13, it is necessary to reduce the contact angles θ1a and θ1b by reducing the outer race inner spherical surface length Lo and the cage width Lc.
FIG. 7 is a perspective view illustrating essential parts of the conventional fixed ball type constant velocity joint for a vehicle to show friction between a cage and an inner race when a joint angle is created.
As shown in FIG. 7, when a joint angle of the inner race 15 is created by θ, contact points of the inner spherical radius R32 of the cage 14 and the outer spherical radius R22 of the inner race 15 are generated at ends of an inner race width Li and an inner diameter part of the cage width Lc due to the cage axial ball division force Fc and a tolerance between the two components, that is, between the cage 14 and the inner race 15. Here, frictional forces μF2a and μF2b are generated by contact forces F2a and F2b. The frictional forces μF2a and μF2b are proportional to contact angles θ2a and θ2b. That is to say, if the inner race width Li and the cage width Lc are increased, the contact angles θ2a and θ2b are increased and the frictional forces μF2a and μF2b are also increased. Therefore, in order to reduce the frictional forces μF2a and μF2b between the cage 14 and the inner race 15, it is necessary to reduce the contact angles θ2a and θ2b by reducing the inner race width Li and the cage width Lc.
As described above, in the conventional fixed ball type constant velocity joint for a vehicle, in order to reduce the frictional forces μF1a and μF1b between the cage 14 and the outer race 13 and the frictional forces μF2a and μF2b between the cage 14 and the inner race 15, it is necessary to reduce the contact angles θ1a, θ1b, θ2a and θ2b by reducing the cage width Lc, the outer race inner spherical surface length Lo and the inner race width Li. However, the reducing of the cage width Lc, the outer race inner spherical surface length Lo and the inner race width Li has limitation in view of joint strength and structure.